In recent years, as electronic devices for microprocessor (CPU) and the like are made to have higher performance and to be smaller, heat generated by the electronic devices has been increasing. Accordingly, a cooling technique having a high cooling capacity for electronic devices is demanded. Generally, an electronic device is cooled down by use of a radiator part, such as a heat sink. An electronic device and a radiator part are joined with each other by a highly thermal conductive material so that thermal resistance can be reduced at the joint (gap).
Surfaces of an electronic device and a radiator part are not completely flat and have minute bumps. Accordingly, in the case of using a hard heat transfer material, such as metal, at a joint gap between the electronic device and the radiator part, joint (adhesion) between the material and the electronic device or the radiator part is insufficient. As a result, the thermal resistance at the joint is increased. Therefore, currently, a thermal grease, a phase change sheet, and the like which can fill the uneven joint gap without leaving any space therein have been widely used. However, the thermal conductivities of these heat transfer materials are lower than that of a metal material (in many cases, <<10 W/mK). Accordingly, there arises a problem of having a higher thermal resistance when the joint gap is thicker.
Japanese Patent Application Publication No. Hei 8-238707 disclose a radiating sheet made of a silicone rubber substrate with an additive agent, such as a metal powder. U.S. Pat. No. 6,651,736 discloses a method of adding as a filling agent a carbon material to a binder such as grease and polymer. However, even when these methods are adopted, the thermal conductivity of a whole system is under constraint of the thermal conductivity of a substrate (binder). Accordingly, it is impossible to sufficiently reduce thermal resistance.
Japanese Patent Application Publications No. Hei 10-56114 and No. Hei 11-302545 and U.S. Pat. No. 5,695,847, disclose sheet-like heat transfer parts in which graphite fibers and the like are orientated perpendicularly to a joint plane. A carbon-based composite part obtained in this method exhibits high thermal conductivity in a bulk state. However, such a heat transfer part allows generation of a space in a joint part as similar to the case of using the above-described hard heat transfer material, such as metal. As a result, the thermal resistance in the entire joint gap including the space is increased.    [Patent Document 1] Japanese Patent Application Publication No. Hei 8-238707    [Patent Document 2] U.S. Pat. No. 6,651,736    [Patent Document 3] Japanese Patent Application Publication No. Hei 10-56114    [Patent Document 4] Japanese Patent Application Publication No. Hei 11-302545    [Patent Document 5] U.S. Pat. No. 5,695,847